An organic electroluminescent device (hereinafter referred to as organic EL device) in the simplest structure is generally constituted of a light-emitting layer sandwiched between a pair of counter electrodes and functions by utilizing the following phenomenon. Upon application of an electrical field between the electrodes, electrons are injected from the cathode and holes are injected from the anode and they recombine in the light-emitting layer with emission of light.
In recent years, organic thin films have been used in the development of organic EL devices. In particular, in order to enhance the luminous efficiency, the kind of electrodes has been optimized for the purpose of improving the efficiency of injecting carriers from the electrodes and a device has been developed in which a hole-transporting layer of an aromatic diamine and a light-emitting layer of 8-hydroxyquinoline aluminum complex (hereinafter referred to as Alq3) are disposed in thin film between the electrodes. This device has brought about a marked improvement in the luminous efficiency over the conventional devices utilizing single crystals of anthracene and the like and thereafter the developmental works of organic EL devices have been focused on commercial applications to high-performance flat panels featuring self-luminescence and high-speed response.
In another effort to enhance the luminous efficiency of the device, the use of phosphorescence in place of fluorescence is investigated. The aforementioned device comprising a hole-transporting layer of an aromatic diamine and a light-emitting layer of Alq3 and many other devices utilize fluorescence. The use of phosphorescence, that is, emission of light from the excited triplet state is expected to enhance the luminous efficiency approximately three to four times that of the conventional devices utilizing fluorescence (emission of light from the excited singlet state). To achieve this objective, the use of coumarin derivatives and benzophenone derivatives in the light-emitting layer has been investigated, but these compounds merely produced luminance at an extremely low level. Thereafter, europium complexes were tried to utilize the excited triplet state, but failed to emit light at high efficiency. In recent years, as is mentioned in the patent document 1, a large number of researches are conducted with the objective of enhancing the luminous efficiency and extending the service life while mainly utilizing organic metal complexes such as iridium complexes.                Patent document 1: JP2003-515897 A        Patent document 2: JP2001-313178 A        Patent document 3: JP2006-76901 A        Patent document 4: WO2005-76668 A        Patent document 5: WO2003-78541 A        Non-patent document 1: Applied Physics Letters, 2003, 83, 569-571        Non-patent document 2: Applied Physics Letters, 2003, 82, 2422-2424        
A host material to be used with the aforementioned dopant material becomes important in order to enhance the luminous efficiency. Of host materials proposed thus far, a typical example is 4,4′-bis(9-carbazolyl)biphenyl (hereinafter referred to as CBP) presented in the patent document 2. CBP exhibits relatively good luminous characteristics when used as a host material for green phosphorescent emitters, typically tris(2-phenylpyridine)iridium (hereinafter referred to as Ir(ppy)3). On the other hand, CBP fails to perform with sufficient luminous efficiency when used as a host material for blue phosphorescent emitters. This is because the energy level of the lowest triplet excited state of CBP is lower than that of common blue phosphorescent emitters and the triplet excitation energy of a blue phosphorescent emitter in use is transferred to CBP. That is to say, if a phosphorescent host material were designed to have triplet excitation energy higher than that of a phosphorescent emitter, the triplet excitation energy of said phosphorescent emitter would be confined effectively and, as a result, the luminous efficiency would be enhanced. With the objective of improving this energy-confining effect, the triplet excitation energy is increased by modifying the structure of CBP in the non-patent document 1 and the luminous efficiency of bis[2-(4,6-difluorophenyl)pyridinato-N,C2′]iridium picolinate (hereinafter referred to as Flrpic) is improved by this means. Similarly, the luminous efficiency is enhanced by using 1,3-dicarbazolylbenzene (hereinafter referred to as mCP) as a host material in the non-patent document 2. However, these host materials are not satisfactory in practical use, particularly from the viewpoint of durability.
Moreover, the host material needs to have the balanced electrical charge (hole and electron) injection/transport characteristics in order to enhance the luminous efficiency. The electron transport property is inferior to the hole transport property in the case of CBP and this disturbs the balance of electrical charges in the light-emitting layer and causes excess holes to flow out to the side of the cathode thereby reducing the probability of recombination of holes and electrons in the light-emitting layer and decreasing the luminous efficiency. Furthermore, occurrence of the recombination is limited to a narrow region in the vicinity of the interface on the cathode side in this case. Therefore, in the case where an electron-transporting material like Alq3 whose energy level of the lowest triplet excited state is lower than that of Ir(ppy)3 is used, there may also arise the possibility that the luminous efficiency may decrease due to transfer of the triplet excitation energy from the dopant to the electron-transporting material.
One of the means to prevent holes from flowing out to the electron-transporting layer is to provide a hole-blocking layer between the light-emitting layer and the electron-transporting layer. This hole-blocking layer accumulates holes efficiently in the light-emitting layer and contributes to improve the probability of recombination of holes and electrons in the light-emitting layer and enhance the luminous efficiency (the patent document 2). Hole-blocking materials in general use include 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (hereinafter referred to as BCP) and p-phenylphenolato-bis(2-methyl-8-quinolinolato)aluminum (hereinafter referred to as BAlq). These materials can prevent holes from flowing out of the light-emitting layer to the electron-transporting layer; however, the lowest energy level of the excited triplet state of both of them is lower than that of a phosphorescent dopant such as Ir(ppy)3 and sufficient luminous efficiency cannot be obtained.
Moreover, BCP tends to crystallize even at room temperature and lacks reliability as a hole-blocking material and the life of the device is extremely short. Although BAlq is reported to have a Tg of approximately 100° C. and provide the device with relatively good life, its hole-blocking ability is not enough.
The aforementioned examples indicate that, in order for organic EL devices to perform at high luminous efficiency, a host material is required to have high triplet excitation energy and to be balanced in the electrical charge (hole and electron) injection/transport characteristics. Furthermore, the host material is hopefully a compound endowed with electrochemical stability, high heat resistance, and excellent stability in the amorphous state. However, no compound capable of satisfying these properties on a practical level has been known at the present time.
Attempts have been made to introduce a skeleton that has an excellent hole transport property as represented by a triarylamine or carbazole and another skeleton that has an excellent electron transport property as represented by pyrimidine or triazine into one and the same molecule.
The following compounds are disclosed in the patent document 3.

In these compounds, the lowest unoccupied molecular orbital (LUMO) extends over the triazine ring or the pyrimidine ring at the center of the molecule and the periphery of the LUMO is enveloped in the carbazole skeletons wherein the highest occupied molecular orbital (HOMO) extends. The LUMO is thus centralized and confined inside the molecule and this arrangement acts against the movement of electrons between molecules and, further, it necessitates improvement in stability when the molecule is subjected to one-electron reduction.
Moreover, the following compounds are disclosed in the patent documents 4 and 5.

In these compounds too, the LUMO extends over the two rings consisting of the benzene ring and the pyrimidine ring at the center of the molecule and the periphery of the LUMO is enveloped in the carbazole skeletons wherein the HOMO extends. That is, the LUMO is confined inside the molecule and this arrangement acts against the movement of electrons between molecules. Furthermore, in these compounds, bonding of the carbazolyl group to the benzene ring lowers the triplet excitation energy. Therefore, depending upon the triplet energy level of the dopant in use, the triplet excitation energy of the dopant is transferred to the host molecule and this makes it impossible to obtain high luminous efficiency.
Further, the following compounds are given as examples in the patent document 5.

In these compounds too, the LUMO is centralized on the two rings, either on the benzene ring and the pyrimidine ring or on the benzene ring and the triazine ring, at the center of the molecule and the periphery of the LUMO is enveloped in the carbazole skeletons wherein the HOMO extends. That is, confinement of the LUMO inside the molecule acts against the movement of electrons between molecules. In these compounds too, bonding of the carbazolyl group to the benzene ring lowers the triplet excitation energy. In consequence, depending upon the triplet energy level of the dopant in use, the triplet excitation energy is transferred to the host molecule and this makes it impossible to obtain high luminous efficiency.
When a plurality of skeletons differing from one another in the electrical charge transport properties are introduced into one and the same molecule, the molecule may undergo large changes in the balance of electrical charges and in the stability depending upon the method used for the introduction. This indicates that the use of a molecule of this kind particularly as a host material is bound to affect vitally the luminous characteristics and life of the device and necessitates further improvements when practical use is intended.